Every four years we get to experience the phenomenon known as Leap Year. One additional day is added to the calendar to account for the fact that it takes about 365.25 days for the earth to make a full orbit of the sun. Here’s a pop quiz to test your knowledge about Leap Year. What is the date of the extra day we get on those special years?

Did you answer February 29? If so, you just failed the test. The actual Leap Day, contrary to what you might expect, is February 24. Why is that? Read on, and learn more about the intricacies of Leap Year.

The Julian calendar was a great step forward when it was adopted by Julius Caesar in 45 BC. Using the best scientific observations of the day, it worked off of the assumption that it takes 365.25 days for the earth to travel around the sun. With that in mind, it used a 365-day year for three years and a 366-day year every fourth year. That year, of course, is what we call Leap Year.

The question then arose as to when that extra day should be added. Since the start and end of the months had significance for Rome’s religious festivals, Caesar was wary of doing anything that could interfere with associated religious practices. Instead, he decreed that the day should come at the conclusion of the festival of Terminalia. That day was the 24th of Februarius (February 24).

Initially, no extra day was added. Instead, 24 Februarius was to be a 48-hour day every fourth year, before moving on to 25 Februarius. At that point, the regular 24-hour day would continue for the next four years. This practice led to confusion, and the official count of days eventually conformed with the popular practice of treating the 48-hour period as two days. The extra day, however, was still considered to be 24 Februarius, simply pushing the remaining days of the month back by one.

As you can imagine, this created some confusion. Writing in the early fifth century, Macrobius gave this account:

Caesar’s regulation of the civil year to accord with his revised measurement was proclaimed publicly by edict, and the arrangement might have continued to stand had not the correction itself of the calendar led the priests to introduce a new error of their own; for they proceeded to insert the intercalary day [Leap Day], which represented the four quarter-days, at the beginning of each fourth year instead of at its end, although the intercalation ought to have been made at the end of each fourth year and before the beginning of the fifth.

This error continued for thirty-six years by which time twelve intercalary days had been inserted instead of the number actually due, namely nine. But when this error was at length recognized, it too was corrected, by an order of Augustus, that twelve years should be allowed to pass without an intercalary day, since the sequence of twelve such years would account for the three days which, in the course of thirty-six years, had been introduced by the premature actions of the priests.

Despite the confusion about Leap Day, the Julian calendar was a marvel of its time and greatly improved upon its predecessors. It was not perfect, however. The calendar assumed a 365.25-day year. In reality, an actual solar year is 365.24219 days. This led to a gradual deviation from the calendar date and the observed astronomical date.

The Council of Nicaea (A.D. 325) decreed that the date for Easter should be the first Sunday after the first full moon following the Spring equinox. By the 1500s, the tiny miscalculations of the Julian calendar had added up to the point where the measuring point for the Spring equinox was off by about 12 days, throwing the proper observance of Easter askance. Pope Gregory XIII took matters in hand to correct this by instituting a revision of the Julian calendar. As countries adopted the new calendar, they had to grapple with an unusual period of time where events might be recorded on two different dates. That is why George Washington had two different birthdays. Gregory’s revision — which happens to be the one most of the world follows today (unless, of course, you are in North Korea) — kept everything about the Julian calendar in place, except for a slight modification of Leap Year.

Instead of Leap Year occurring every four years, there is one rare exception. The new rule is that every year that is exactly divisible by four is a Leap Year, except for years that are exactly divisible by 100, but these centurial years are Leap Years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not Leap Years, but the years 1600 and 2000 are.

What is significant is that Gregory only changed the years in which a Leap Day would be added; he did not change which day is designated as such. Consequently, although most of the world thinks that February 29 is the extra day in a Leap Year, it remains February 24.

Of course, there are always exceptions, when it comes to calendars. Two nations, Sweden and Finland, enacted laws to formally designate February 29 as the added day. As for the rest of you who observe the Gregorian calendar, start making your plans now to celebrate February 24 as a gift of one extra day to do with as you please.

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